<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var result = parseInt(prompt("请输入一个三位数"));
        if (result < 100) {
            alert("必须是三位数");
        }else{
            let a = parseInt(result / 100);//百位
            let b = parseInt(result / 10 % 10);//十位
            let c = parseInt(result % 10);
            alert(a+b+c);
        }
    </script>
</body>
</html>